The present invention relates generally to desulfurization of hot coal-derived gases, and more particularly to a method wherein a solid sorbent of zinc ferrite is regenerated with a stream of oxygen-containing gas and steam and with the resulting regeneration tail gas being conveyed into the gasifier for reacting with a calcium compound therein to form environmentally stable calcium-sulfur compounds.
The production of volatile combustion gases by the gasification of coal is becoming of increasing importance as a viable alternative energy source. One of the primary problems encountered during the gasification of coal is the generation of environment polluting sulfur compounds such as hydrogen sulfide and carbon disulfide from the sulfur in the coal.
Efforts to reduce the concentration of deleterious sulfur compounds in the coal-derived gases have been generally successful. One of the sulfur-reducing practices used is to introduce a calcium compound such as lime, limestone or dolomite into the gasifier along with the coal to capture a sufficient percentage of gaseous sulfur produced during the coal-gasification process to reduce the concentration of hydrogen sulfide in the stream of coal-derived gases between 50 and 90%. In such gasification processes the calcium-to-sulfur ratio is about 1.0 to 5.0 for the primary reduction of the concentration of hydrogen sulfide in the product gases. If desired, residual hydrogen sulfide in the product gases can be removed by suitable condensation and scrubbing techniques with the resulting product gases being essentially sulfur free and environmentally acceptable.
One of the problems attendant with known sulfur-capturing processes such as described above is that the scrubbing systems utilized for removing essentially all the sulfur from the product gases require extensive cooling of the coal-derived gases so as to reduce the desirability of the process for use in systems where the sensible heat in the product gases can be utilized to increase the efficiency of the system. Further, the use of condensing and scrubbing systems often provide a sulfur-containing product which is in itself difficult to dispose of due to environmentally pollution problems.
More recently developed mechanisms for removing hydrogen sulfide and other sulfur compounds from the coal-derived gases, without the cooling associated with the described scrubbing techniques, are the use of solid sorbents such as iron oxide, zinc ferrite or a combination thereof. Zinc ferrite has been found to be a particularly effective sorbent in that it can scavenge virtually all of the hydrogen sulfide in the product gases in contact therewith so as to provide product gases with less than about 10 ppm sulfur and be environmentally acceptable. A shortcoming in utilizing zinc ferrite, iron oxide or combinations thereof is that the saturated sorbents may be regenerated by contacting the hydrogen sulfide-containing absorbent with a stream of steam and oxygen or an oxygen-bearing gas such as air for converting the hydrogen sulfide to sulfur dioxide, but the sulfur dioxide must, in turn, be disposed of.
Gasification systems such as fixed bed, fluidized bed, or moving bed types including pressurized versions are usually configured to have an upper gasification or reducing zone and a lower combustion or oxidation zone. In these gasification systems sulfur capture occurs primarily in the lowermost portion of the reducing zone and the underlying combustion zone. It has been found that in such systems, at temperatures below about 1250.degree. F., which are often present in the upper portion of the reducing zone, that equilibrium sulfur captured by calcium compounds or solid absorbent in particulate form varies from 1 to 50 percent, which is not sufficient to meet environmental requirements. Thus, in such coal gasification systems the sulfur content in the product gases even when the gasifier contains sulfur-capturing materials such as calcium compounds or solid absorbents is often above the level considered to be environmentally acceptable. Thus, the product gases must generally be further processed through the scrubbing and condensing systems or beds of solid absorbent downstream of the gasifier, and as pointed out above, the utilization of such gas clean-up systems downstream of the gasifier have some drawbacks which detract from their desirability for sulfur removal.